The present invention relates generally to an interface coating for use in combination with solid state electronic components or devices, and more particularly to a semisolid interface coating which is adapted to be interposed along a heat dissipating path between a solid state electronic device and a mounting surface. The interface coating of the present invention comprises, in admixture, a polyorganosiloxane graft polymer with a methylsiloxane host together with a linear hydrocarbon having a single terminally positioned unsaturated linkage in the carbon chain, and with a chain length preferably between 16 and 19 carbon atoms. While other such linear hydrocarbons may be utilized, the graft polymer is advantageously prepared from octadecene and a methylsiloxane host, with the graft polymer thereafter being blended with a particulate solid. The particulate solid or filler functions as a thermally conductive component for the blend, and with its small particulate size and high surface area, functions also as a viscosity stabilizer for the polymer component. The polyorganosiloxane graft polymers utilized in the formulations of the present invention may have a melting point ranging from between about 30 degrees C. and 90 degrees C., and preferably having a melting point ranging from between about 30 degrees and 60 degrees C. The melting point range of an individual polymer is relatively narrow and sharply defined, thereby contributing to the consistency of performance.
The polyorganosiloxane graft polymer portion of the interface coating changes phase and becomes at least partially liquid during normal operation of the solid state electronic device with which it is operatively thermally coupled. The interface, when even only partially molten, provides a highly thermally conductive path substantially free of entrapped air so as to facilitate and enhance heat dissipation from the solid state device. The coatings of the present invention may be applied onto the surfaces upon which the solid state electronic device is either being directly mounted, or alternatively, to which the device is being operatively thermally coupled. The coatings of the present invention may be applied as required onto either surfaces of metallic or polymeric materials. These coatings may be applied as continuous layers or may be applied on an intermittent or segmented pattern basis in the form of strips or dots. The coatings may also be die cut and applied to the appropriate surface.
Solid state electronic devices or components are in wide use and application, being incorporated in electronic systems generally, including systems used in the fields of data processing, communications, power supply systems, among others. Solid state electronic devices including power transistors, power modules including converters such as AC-to-DC and DC-to-DC and other similar components. The term "solid state electronic devices" is being used herein in a comprehensive sense, and is intended to include solid state circuits wherein a complete circuit is formed from a single block or chip of semiconductor material, solid state circuit elements such as Zener diodes, silicone controlled rectifiers, as well as other solid state components such as transistors and diodes. Other devices falling within the comprehensive meaning of the term includes passive components, thermoelectric devices, as well as lasers, each of which typically require contact with a heat exchanger or a thermally conductive path for heat dissipation. These devices are typically incorporated in packages designed for mounting on a chassis in accordance with the individual requirements of the specific circuit. As power and frequency requirements increase, and as the space available for these devices or components shrink, these packages typically require highly efficient, effective, and reliable means for dissipating heat created by the solid state electronic devices during periods of normal operation, with the heat typically being transferred by thermal conduction from the package to a mounting surface. This thermal conduction may be undertaken either directly, as occurs when the device is mounted upon the heat dissipating surface, or indirectly as occurs when the device is mounted to a surface which is arranged along a thermal path to a heat dissipating member.
In the past, during assembly, it has been common to apply a layer of grease, typically a silicone grease, or a layer of an organic wax to aid in creating a low thermal resistance path between the opposed mating surfaces of the package and the mounting surface. A layer of such grease is typically applied between these mating surfaces in order to facilitate and enhance thermal conductivity. In certain applications, heat spreaders may be employed along the thermal path to achieve certain heat dissipating objectives, and the interface coatings of the present invention may be employed along the surfaces of heat spreaders as well. In each of these instances, the quantity of entrained air is reduced and thermal conductivity is improved in order to increase thermal energy flow from the electronic device.
In the course of a typical assembly operation prior to the present invention, silicone grease, for example, was applied liberally to the surfaces so as to assure its presence in all of those locations where it is reasonably expected to be needed or required. As a result, the assembly operations utilizing grease typically over-apply the material, and the products become messy, with the grease in certain instances, interfering with subsequent assembly operations and also with subsequent operation of the solid state device. Under application has also presented some problems, particularly regarding consistency. The features of the present invention provide a highly thermally conductive coating which may be applied to surfaces along a thermal path, with the coating having a consistent and uniform thickness which contributes to consistency in performance. Given this property in the thermally conductive coating, greater predictability of performance is available from the semiconductor devices utilized in combination with the coating, with these advantages being obtained without experiencing the problems inherent in applications of silicone grease.